A de SQUID consists of a superconducting ring structure with two Josephson junctions inserted therein. The device enables one to detect a minute change in the magnetic flux in an ultra-high sensitivity. With a sensitivity of being several orders of magnitude higher than conventional magneto-resistance devices or Hall magnetometers dc SQUID is commonly used as a magnetometer and a magnetic field gradiometer with an extremely high sensitivity. Furthermore, if the detector element is adaptively modified, dc SQUID can also be utilized as an extremely sensitive detector unit for an electrical voltage, a current, and a resistance. Consequently it is widely employed for applications in the fields of medical diagnosis, military detection, exploration of underground natural resources, basic research and so on.
Ever since the discovery of the high-Tc superconductors in 1986 many attempts have been made to develop dc SQUID devices using high-Tc superconducting materials. Unfortunately fabricating an artificial tunnel junction, the essential element of a SQUID device, using high-Tc superconducting materials has ever been an enormously difficult task. The difficulty stems from the extremely delicate material properties of high-Tc superconductors; the surface degrades easily and loses the superconductivity over the depth of a few nm from the surface once the surface is exposed. This material properties combined with the extremely short superconducting coherence length of high-Tc superconductors make it very difficult to artificially fabricate high-Tc Josephson tunnel junctions.
Accordingly, Josephson weak links, in which grain-boundary junctions such as step-edge junctions, bi-crystal junctions, and bi-epitaxial junctions are employed, have been alternately used as high-Tc superconducting Josephson junction units for varied applications. However, since such grain-boundary junctions are not true tunnel junctions, ideal Josephson tunneling characteristics cannot be obtained. Such grain-boundary junctions are also vulnerable to thermal recycling so that junction characteristics of the weak links are easily deteriorated. These unfavorable material properties pose most serious obstacles in applying high-Tc superconductors to SQUID devices or any active devices consisting of Josephson junctions.